[AArch64][SVE2] Load/store instruction fixes

Summary:
* Loads and stores in SVE2 are gather/scatter not contiguous, fixed by
  renaming multiclasses to reflect this and also updated comments.
* Remove aliases from load/store multiclasses that reflect the behaviour
  of the original form.
* Fix bug in scatter store implementation, vector list should be used as
  input, not output.

Reviewed By: sdesmalen

Differential Revision: https://reviews.llvm.org/D65392

llvm-svn: 367398